Systematic Orientation.
Terms relating to position are unfortunately used by writers on entomology in various, even in opposite senses. Great confusion exists as to the application of such words as base, apex, transverse, longitudinal. We can best explain the way in which the relative positions and directions of parts should be described by reference to Figure 62. The spot 3 represents an imaginary centre, situated between the thorax and abdomen, to which all the parts of the body are supposed to be related. The Insect should always be described as if it were in the position shown in the Figure, and the terms used should not vary as the position is changed. The creature is placed with ventral surface beneath, and with the appendages extended, like the Insect itself, in a horizontal plane. In the Figure the legs are, for clearness, made to radiate, but in the proper position the anterior pair should be approximate in front, and the middle and hind pairs directed backwards under the body. The legs are not to be treated as if they were hanging from the body, though that is the position they frequently actually assume. The right and left sides, and the upper and lower faces (these latter are frequently also spoken of as sides), are still to retain the same nomenclature even when the position of the specimen is reversed. The base of an organ is that margin that is nearest to the ideal centre, the apex that which is most distant. Thus in Fig. 62, where 1 indicates the front tibia, the apex (A) is broader than the base (B); in the antennae the apex is the front part, while in the cerci the apex is the posterior part; in the last abdominal segment (2) the base (B) is in front of the apex (A). The terms longitudinal and transverse should always be used with reference to the two chief axes of the body-surface; longitudinal referring to the axis extending from before backwards, and transverse to that going across, i.e. from side to side.
CHAPTER IV
ARRANGEMENT OF INTERNAL ORGANS–MUSCLES–NERVOUS SYSTEM–GANGLIONIC CHAIN–BRAIN–SENSE-ORGANS–ALIMENTARY CANAL–MALPIGHIAN TUBES–RESPIRATION–TRACHEAL SYSTEM–FUNCTION OF RESPIRATION–BLOOD OR BLOOD-CHYLE–DORSAL VESSEL OR HEART–FAT-BODY–OVARIES–TESTES–PARTHENOGENESIS–GLANDS.
The internal anatomy of Insects may be conveniently dealt with under the following heads:—(1) Muscular system; (2) nervous system; (3) alimentary system (under which may be included secretion and excretion, about which in Insects very little is known); (4) respiratory organs; (5) circulatory system; (6) fat-body; (7) reproductive system.
Fig. 63.—Diagram of arrangement of some of the internal organs of an Insect: a, mouth; b, mandible; c, pharynx; d, oesophagus; e, salivary glands (usually extending further backwards); f, eye; g, supra-oesophageal ganglion; h, sub-oesophageal ganglion; i, tentorium; j, aorta; k1, k2, k3, entothorax; l1-l8, ventral nervous chain; m, crop; n, proventriculus; o, stomach; p, Malpighian tubes; q, small intestine; r, large intestine; s, heart; t, pericardial septum; u, ovary composed of four egg-tubes; v, oviduct; w, spermatheca (or an accessory gland); x, retractile ovipositor; y, cercus; z, labrum.
Many of the anatomical structures have positions in the body that are fairly constant throughout the class. Parts of the respiratory and muscular systems and the fat-body occur in most of the districts of the body. The heart is placed just below the dorsal surface; the alimentary canal extends along the middle from the head to the end of the body. The chief parts of the nervous system are below the alimentary canal, except that the brain is placed above the beginning of the canal in the head. The reproductive system extends in the abdomen obliquely from above downwards, commencing anteriorly at the upper part and terminating posteriorly at the lower part of the body cavity.
In Fig. 63 we show the arrangement of some of the chief organs of the body, with the exception of the muscular and respiratory systems, and the fat-body. It is scarcely necessary to point out that the figure is merely diagrammatic, and does not show the shapes and sizes of the organs as they will be found in any one Insect.
Muscles.